Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Add a comment describing where to include bsd.dtrace.mk.

Sponsored by: The FreeBSD Foundation

Fix bugs introduced in the previous rev:
* add a phatom target for the DTRACEOBJS
* when invoking DTrace, don't add DTRACEOBJS to the command line.

Sponsored by: The FreeBSD Foundation

Add bsd.dtrace.mk. This allows userland programs and libraries to
define USDT probes on a provider.d file and then use this new make
infrastructure to build the corresponding header file and object file.

This will only take effect when the user defines WITH_DTRACE when building.

Sponsored by: The FreeBSD Foundation